The present invention relates to atomic force microscopes (AFMs), probe tip design for AFMs, deflection mechanism (cantilever arm) design, sample mounting for AFMs, and techniques for the use of AFMs. More specifically, this invention relates to an atomic force microscope particularly useful for scanning small samples to obtain the surface topography of a surface thereof comprising, a cantilever arm supported on one end for movement on an opposite end, the opposite end having a sample holding area thereon carrying a sample to be scanned; a scanning tip; scanning means disposed over the sample holding area and carrying the scanning tip on a scanning end thereof for moving the scanning tip over the sample in a scanning pattern; detector means for detecting movement of the opposite end of the cantilever arm and for developing a signal at an output thereof measuring an amount of movement of the opposite end of the cantilever arm; and, controller and driver means connected to the scanning means and to receive the signal from the detector means for driving the scanning means in the scanning pattern and for deriving surface image data from the signal.
Atomic force microscopes are devices that provide three dimensional topographic images of surfaces. These devices are capable of providing resolution of surface features to atomic dimensions. In an atomic force microscope (AFM), an extremely sharp tip is mounted on a very sensitive spring structure. The tip is usually positioned over a surface to be scanned such that the Van der Waals attraction between the surface and the tip and the repulsive force of the surface on the tip are very close to equilibrium. Thus, the force of the tip on the surface can be extremely low. If the tip is scanned across the sample surface, the deflection of the tip will vary with the surface structure and this modulation produces the AFM image. Alternatively, the sample may be servoed up and down such that the tip deflection and thus the tip force is kept constant. This will be referred to hereinafter as the "feedback mode". The deflection of the tip can be sensed in various ways, such as using the tunneling effect off the backside of the tip or by optical means such as beam deflection or interferometry. Typically, most current AFMs mount the tip on a low spring constant cantilever arm and sense deflection by monitoring the change in angle of reflected light off the backside of the cantilever arm.
A typical prior art AFM is illustrated in FIG. 1. The sample 10 is attached to the top of a three-axis scanner 12 which is typically a piezoelectric tube type scanner of a type well known in the art. The sample 10 is brought into close proximity to a sharp tip 14 that is attached to or part of a small, stiff cantilever arm 16. Some means of detecting the deflection of the cantilever arm 16 is required. An optical means is illustrated, where light 18 from a small laser 20 is focused onto a reflective area on the back of the cantilever arm 16 and the reflected light 18' is detected by a two-element photodetector 22. The difference between the signals from the two elements of the photodetector 22 is determined by the differencing logic 24 and the difference signal therefrom is used by the microscope controller 26 to create the desired image of the surface being scanned as well as provide feedback for scanner control by the scanner driver 28 and other functions well known in the art. The inventors herein typically operate their instruments in the feedback mode; so, vertical scan control is used to derive the image produced. Spring constants on the order of one newton/meter and lengths of 100 microns are typical parameters for the cantilever arm 16. When operated in a constant deflection mode using feedback, very small forces can be applied to the sample 10.
AFMs can operate directly on insulators as well as conductors and, therefore, can be used on materials not directly accessible to other ultra-high resolution devices such as scanning electron microscopes (SEMs) or scanning tunneling microscopes (STMs).
The tip in an AFM must be positioned with extreme accuracy in three dimensions relative to the sample. Motion perpendicular to the sample (z-axis) provides surface profile data. Motion parallel to the surface generates the scanning. In a typical system, the image is developed from a raster type scan with a series of data points collected by scanning the tip along a line (x-axis), displacing the tip perpendicularly in the image plane (y-axis), and repeating the step and scan process until the image is complete. The precise positioning in x, y, and z required to generate atomic scale images is usually accomplished with a piezoelectric device as the scanner 12. Piezoelectric devices can be made to expand or contract by applying voltages to electrodes that are placed on the piezoelectric material. The motions produced can be extremely small, with sensitivities as low as tens of angstroms per volt. The total deflection possible for these scanners is typically less than 100 microns. Scanners with different sensitivities are used for different applications, with low sensitivity used for atomic resolution images and higher sensitivity scanners used for lower resolution, larger area images. The design of piezoelectric scanners, including the shape of the scanner and the placement of electrodes, is well known in the art.
In an AFM, either the sample can be attached to the scanner and the tip held stationary or the tip can be attached to the scanner and the sample fixed. Typically, most existing AFMs scan the sample. As the tip is scanned in x and y, the z axis movement is closely coupled to the tip deflection. In an AFM, either the tip deflection can be monitored as the sample is scanned or the z position can be varied, with feedback, to maintain the deflection constant. Modulating the z position with feedback is useful for minimizing the contact force between the tip and the sample, and also allows the AFM to be used for other measurements, such as stiffness.
Most AFMs use optical means to measure tip deflection. As described above with respect to FIG. 1, one method focuses a light beam onto a reflective surface on the back of the cantilever arm. As the cantilever arm moves up and down in response to the sample topography, the reflected beam moves up and down on a photodetector which develops a signal which is used for image or feedback calculation. Other optical means use interference techniques. In one interference method, a laser diode is placed very close to the cantilever arm such that the light reflected off the cantilever arm is reflected back into the laser diode. The laser power will vary with the relative phase of the reflected light and the output light. The phase is dependent on the distance of the diode from the cantilever arm and, thus, laser power will vary with the cantilever arm deflection. Another interference method is to build a small interferometer using a fixed mirror and the cantilever arm back, and to inject the light into the interferometer with fiber optics.
All existing prior art AFMs mount a probe tip on a cantilever arm arrangement. The tip is then scanned over a sample and the deflection of the cantilever arm provides the surface information. For an AFM to operate, the cantilever arm must exert very small forces on the sample and have reasonably fast response to surface variations. To achieve these requirements, the cantilever arm has to be very small and very stiff. Typical cantilever arms are a few 100 microns long and 10s of microns wide at the tip end. Therefore, the tip which is mounted on the end of the cantilever arm has to be extremely small or its mass will decrease the resonant frequency of the cantilever arm. The resonant frequency is a measure of the stiffness and, therefore, the response time of the cantilever arm-to-surface variations.
The requirement for small mass for AFM tips limits the flexibility in choice of tip material and design. Typically, existing AFM tips have not been made with as small tip angles as have been achieved with STM tips, which are attached to the scanner and are not as constrained as to size and mass. For instance, existing AFM tips have been made from diamond chips, etched silicon, or etched silicon nitride, which are quite stiff and atomically sharp, but which have tip profiles of 30-90 degrees. Also, these tips are not good electrical conductors. Such a tip is shown in an article by K. Wickramasinghe, Scientific American, October 1989, pg 98. By comparison, STM tips made from etched tungsten wire are also atomically sharp, but have tip profiles of 5-10 degrees, and are superior for measuring surface topography which has steep inclines. It would be very difficult, however, to produce an etched tungsten wire that was small enough, could be mounted to a cantilever arm, and be easily replaceable.
Wherefore, it is an object of the present invention to provide an atomic force microscope of the cantilever arm type which is able to employ tips made from etched wire, or the like, without the detrimental effects normally attendant to their large mass.
It is another object of the present invention to provide an atomic force microscope of the cantilever arm type which mounts the sample on the cantilever arm and mounts the tip separately.
It is still another object of the present invention to provide an atomic force microscope of the cantilever arm type which is particularly suited for scanning small samples.
It is yet another object of the present invention to provide a microscope which can operate either as an atomic force or tunneling microscope; or, as a combination.
Other objects and benefits of this invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.